1. Field of the Invention
The present invention relates to the treatment of contaminated soil and, more particularly, pertains to an in-situ/ex-situ method and apparatus suitable for bioremediation of contaminated soils.
2. Description of the Prior Art
In the past, soil washing or flushing has been applied to water soluble contaminants, but for less water soluble hydrocarbons, surfactants added to the wash solutions can help dissolve the contaminants. Recovery of wash solution and contaminant is typically accomplished by the "pump and treat" method. Sometimes, groundwater contamination occurs because of poor recovery or lateral and vertical seepage during an in-situ soil washing operation.
It is also know to exploit microorganisms to detoxify or degrade contaminants. This treatment method is known as bioremediation.
Bioremediation may be effected under aerobic and anaerobic conditions. Major requirements for effective bioremediation are: a biodegradable organic substrate, an appropriate and active microbial community (consortium), bioavailabilty of the polluting substrate to the microorganisms, and the creation of optimal conditions for microbial metabolism. Sometimes bioremediation requires further biostimulation with nutrients or some specific analogue substrate; it may also require bioaugumentation of the microbial community if the site does not have an appropriate indigenous biodegrading population.
The biodegradation of the contaminant is effected by complete mineralization or biotransformation into non toxic, less toxic, or more toxic daughter compounds. Sometimes the biotransformation by-products polymerize and or react with humid substances to become recalcitrant and therefore persist in the environment for a long time.
Land-farming is the simplest aerobic biodegradation technique where the contaminated soil is spread on an agricultural field for biodegradation. Its draw back is the difficulty in optimising the performances of the microorganisms; there is also the possible contamination of subsoil by leachate. This method also requires excavation, transportation, etc.
Other popular aerobic and anaerobic methods are mostly effected on prepared beds or in tank reactors.
In prepared beds, cell surfaces are lined with impermeable barrier boundaries before placing the contaminated soil in the cells. The soil is also conditioned by adjusting the pH and the nutrient status to optimal levels suitable for biodegrading the contaminants. Subsequent addition of supplementary nutrients to the soil may be carried out by sprinkling. Oxygen supply to the biopile is often by diffusion, aided by frequent tilling. Sometimes the prepared bed is instrumented with a network of pipes that receive drain effluent; these pipes are sometimes used for aerating the biopiles as well. Biopiling may be a treatment or a biopreventive measure; when used for prevention, there is usually no impermeable lining.
U.S. Pat. No. 4,850, 745 issued to Hater et al. on Jul. 25, 1989 teaches located prepared beds below storage tanks, which contain viable or dormant organisms capable of degrading the organic compounds of interest. Vertical pipes supply nutrients or nutrient vapours, including air and steam, to the prepared bed located below the ground. Distribution of nutrients and air in the contaminated zone is accomplished by a vacuum, applied on or close to the soil surface. The ground water is not protected from receiving seepage during nutrient addition, or if there is a rainfall event occurring within the treatment period.
This method of nutrient and air supply, via pipes, to prepared bed or biopile is also applicable to bioventing. In bioventing vertical pipes carry nutrient vapour and air into the contaminated subsoil region below the contaminated zone, while vacuum suction applied to extraction wells, at different space intervals, forces the nutrient and air to diffuse across the contaminated region, before being drawn upwards. As the nutrient vapours and air (or steam) are infusing through the soil, low boiling hydrocarbons are stripped and recovered via extraction wells. All nutrients and additives can not be supplied in the vapour phase. This poses a limitation to in-situ bioventing since non vapour nutrients are also required to biostimulate and to create optimal conditions for the biodegrading indigenous microorganisms. Another limitation, envisaged in bioventing, is the poor bioavailability of the organic contaminant to the microorganisms.
Bioavailability of contaminants to biodegrading organisms can be increased by allowing sufficient contact time between the contaminant and the microorganisms. In the presence of optimal moisture content, some microorganisms produce biosurfactants that bring the organic compound into solution. Optimum moisture content, as in a bench slurry microcosm study, should make both transport and metabolism easy. Microbial uptake of substrate is more efficient in solution. However, in bioventing, poor contact time and insufficient dissolution of contaminants results in poor bioavailability of contaminants to the microbial degraders.
It is also known to have an in-situ bioremediation apparatus that can inject nutrient fluids periodically and oxygenated fluid continuously, via horizontal or vertical pipes installed below a contaminant plume. The fluids are then drawn upwards or horizontally under suction, across the plume, so that the nutrient fluid stimulates growth of the indigenous microorganisms within the plume. The nutrient fluid is essentially methane or propane, utilized by methanotrophs. Other nutrients can also be incorporated into the nutrient fluid. The nutrient fluid specifically causes an increase in the indigenous methane degrading population. Afterwards, the nutrient fluid is stopped allowing the increased population to starve and consequently forcing the organisms to cometabolize the contaminants in the plume. Constituents of the oxygenated fluid may be: air or oxygen-nitrogen mixtures, water vapour, or steam.
Although the above-described apparatus is effective in some particular applications, it does not make provision for the protection of groundwater from leachate which are likely to result from a combination of rainfall and treatment of the soil above the ground water.
In another in-situ bioremediation method, the nutrients were introduced via pipes and they percolated through the soil profile based on the geological gradient of the terrain. The percolating nutrients, and sometimes metabolic byproducts from contaminants, were recovered downstream via extraction wells. Since this method relies on the geological gradient, nutrient distribution may be uniform because physical obstructions can divert the flow of the percolating nutrient away from densely contaminated regions. Some nutrients, including phosphates, are needed to optimize in-situ bioremediation and are not easily mobile under normal gradient flow. As well as the poor distribution of nutrients in the soil for in-situ bioremediation, oxygen supply also limits microbial degradation of hydrocarbons in the soil.
In-situ bioremediation has had limited efficiency because of ethical and regulatory constraints as well as technological limitations.
In view of the foregoing, there is a demand for a delivery system adapted to uniformly deliver nutrients, including, oxygen or surfactant or microbes, in the soil profile so that microbial metabolism may be accelerated, thus effecting a successful bioremediation.